I used never to study or do homework or anything. Completely relied on what I managed to pick up in class and general knowledge, most of junior and preliminary schoolwork is pretty straightfoward anyway. I put effort into assignments and in class assessments though, so my marks and reports and stuff have always been quite good, even though a lot of teachers told me to start a study habit.
Now I'm in yr 12 I'm slowly starting to discover that you can't just get away with relying completely on natural intelligence. This has been a big shock for me cause I've been doing that since I started primary school.
I still don't really study, at least not in the conventional sense - I do make notes, but I don't set aside time every night to study them. What I try to do instead is try to make sure that I've fully absorbed what we were taught in class, and that I'm finishing all homework and essays set. I also make sure that I have a syllabus doco for each subject, and when I make notes, it's under syllabus dot-points so that I'm not learning a whole lot of info that I don't need (even if the stuff you don't need is way more interesting)
I think a lot of people who are naturally very smart are at a bit of a disadvantage, cause they often take their intelligence for granted and use it as an excuse to slack off and not do much work. Then when the hsc rolls around they realise that they've been idiots and then they PANIC. This has happened to a lot of people I know, but it is just a personal opinion.
Basically I think that no matter how smart you are everyone needs to work hard to get the mark they want in the hsc. Having said that, acing the hsc isn't required to live a happy life.